Many types of propeller-driven aircraft are equipped with systems which can change the pitch of the propeller for reversing thrust. Thrust reversal provides braking power after the aircraft touches down during a landing maneuver. In addition, reverse thrust can be used for motive power on the ground, as in backing the aircraft away from an airport terminal.
Thrust reversal is illustrated in FIGS. 1-3. FIG. 1 shows a typical aircraft propeller 103 having blades 104, and FIG. 2 is a view taken along lines 2-2 in FIG. 1. Pitch is defined as the angle B made between the chord 105 of the propeller blade and circumference 106, also shown in FIG. 1.
Blade 104 in FIG. 2 illustrates a positive pitch angle, used during forward flight. The propeller 103 rotates in a direction shown by arrow 109 in FIGS. 1 and 2 and the incoming airstream 112 in FIG. 2 is driven roughly along path 113, providing forward thrust for the airplane.
For reverse thrust, the blade 104 is rotated to position 104B in FIG. 3, having a negative pitch angle B. The direction of propeller rotation, shown by arrow 109 in FIGS. 1 and 3, stays the same, so that now the incoming airstream 112 is driven along path 114, providing reverse thrust.
Typically, the sequence of events taken by the pilot during thrust reversal is the following: First, the pilot changes the throttle lever position in order to reduce fuel flow to the engine in order to reduce engine speed. Then, the pilot reverses the pitch of the propeller to that as shown in FIG. 3. Following this, the pilot restores the throttle lever to a position of higher fuel flow in order to resume a higher engine speed. At this time, the pilot monitors a tachometer which indicates engine speed in order to assure that neither the speed of the engine nor that of the propeller becomes excessive.
One hazard associated with the thrust reversing procedure just described results from the fact that the load which the propeller presents to the shaft 116 in FIG. 1 is a function of pitch angle B. For example, large pitch angles, in both forward and reverse thrust, require more shaft horsepower to be delivered by shaft 116 in order to maintain a given rotational speed than do small pitch angles: the larger pitch angles cause a larger Ioad. Further, very small pitch angles, at or near zero degrees (termed flat pitch), present such a small load that it is possible that the propeller might achieve an overspeed condition, inflicting damage upon the engine or propeller. The pilot must monitor engine speed in order to guard against overspeeding resulting from flat pitch.
In a different type of propulsion system, namely, one using contrarotating propellers, such as propellers 117 shown in FIG. 6, additional phenomena occur during thrust reversal. One phenomenon can be explained with reference to FIG. 5, which is a schematic cross- sectional view of the propulsion system in FIG. 6. In that figure, a high-energy gas stream 118 provided by a gas generator (not shown) drives turbines 119 and 121 in opposite rotational directions. Blades 117F and 117A are directly connected to turbines 119 and 121, and also rotate in opposite directions. One such propulsion system is that contained in U.S. patent application "Counter-Rotation Power Turbine," Ser. No. 782,466, filed by K.O. Johnson on May 1, 1985, and which is hereby incorporated by reference.
During forward flight, the propellers accelerate incoming air 112 in the aftward direction indicated by arrow 124, thus providing thrust. However, during thrust reversal, the incoming air 112 is reversed in direction as shown by arrow 126, and the situation causes the fore propeller 117F to be more heavily loaded than the aft propeller 117A, thus inclining the aft propeller to spin faster than the fore propeller, for reasons which will now be explained.
The fore propeller 117F can be viewed as extracting air from region 128, located between the propellers, and pushing it into the incoming airstream 112. The fore propeller 117F thus creates a low pressure in region 128 and a high pressure at region 130. Restated, the fore propeller is propeller 117A is driving air indicated by arrow 133 into a lower pressure region, namely into region 128, as compared with region 130 for the fore propeller. The aft propeller 117A is doing less work and so tends to spin faster.
Stated another way, the fore propeller 117F shields the aft propeller from the incoming airstream 112 so that the aft propeller 117A not need fight this incoming air. Accordingly, the aft propeller is loaded less than the fore propeller, and tends to rotate faster.
Therefore, in such contrarotating systems, not only does the possibility of overspeeding as a result of flat pitch arise, but also the differential loading placed upon the propellers can cause the more lightly loaded propeller to attain an overspeed condition.